Investments in Partially Owned Entities - Related Party Transactions (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2019
Fifth Avenue and Times Square JV \| Joint Venture
Related Party Transaction [Line Items]
Property management fee agreement percentage of gross revenue		2.00%
Development fee as percentage of development costs		1.50%
Vornado Realty Trust \| Fifth Avenue and Times Square JV \| Joint Venture
Related Party Transaction [Line Items]
Development fee as percentage of development costs		5.00%
Property management fee revenue	$ 1,104	$ 1,934
BMS cleaning fees \| Majority-Owned Subsidiary
Related Party Transaction [Line Items]
Revenue from related parties	$ 1,161	$ 1,952

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=116637345&loc=SL114874292-224272

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration